Residential, commercial and industrial structures typically house some sort of combustion sources, such as a furnace and hot water tank, to provide heat and hot water to the occupants. Such combustion sources require air for combustion, and a ventilation in system to vent the resulting gas fumes or exhaust out of the structure. The hot exhaust will usually rise up the exhaust outlet or chimney of the ventilation system and exit to atmosphere.
However, as is well known, a down-draft, or back-draft, in the exhaust outlet may impede or even reverse the travel of the exhaust through the ventilation system. A down-drafts might arise due to atmospheric conditions, or where a "negative" air pressure (i.e. an air pressure lower than that outside the structure) is created within the structure during operation of another combustion source having another exhaust outlet. The most common example of the latter is in residential settings during operation of a fireplace having its own chimney. A fire in a fireplace will typically consume large quantities of indoor air which, if a window is not opened, will often be replaced by fresh air drawn into the residence by a down-draft through the ventilation system servicing other combustion sources, such as the furnace and hot water tank. Should the furnace and/or the hot water tank begin operation during such down-draft, the resulting exhaust may not be sufficient to create a proper up-draft in chimney, and so may be forced to escape into the utility room or other areas of the structure through an air inlet near the bottom of the chimney, through the exhaust source itself, or by another route. Such escape may cause smoke damage to contents within the structure, and may create a dangerous and potentially lethal situation if the fumes are toxic, such as carbon monoxide poisoning.
Prior art systems employ hinged doors or flaps (i.e. dampers) at the air inlet which are urged closed in a down-draft, but dampers do not always seal correctly and have been ineffective.
What is therefore desired is a novel ventilation system which overcomes the limitations and disadvantages of existing systems. Preferably, it should provide for a venting means to induce a positive air flow up an exhaust conduit to prevent down-drafts during operation of exhaust sources and to urge exhaust through the ventilation system to the ambient. In particular, the venting means should include a device for creating a venturi effect to help draw the exhaust away from the exhaust sources and channel the exhaust along the ventilation system toward the ambient. Preferably, the venting means should be controlled to operate only when desired, namely during operation of the exhaust sources when fumes must be vented. The ventilation system should also provide the option of channeling fresh air from the ambient to the venting means and exhaust sources so as not to deplete air within the structure and avoid negative air pressure therewithin.